With the general objective of clarifying some factors involved in the pathogenesis of disease due to infection both in vivo and in vitro studies are being conducted with several intracellular parasites. A reproducible animal model of leishmanial infection with L. tropica was established in a genetic inbred strain of mice and attempts are underway to determine critical features of the parasite with mouse macrophages in vitro. A coccidian originally thought to be Sarcocystis muris, but now considered to be a new species related to toxoplasma (proposed Hammodia hammondi), could not be infective for cats. Work has continued on characterizing a circulating antigen in the blood of animals experimentally infected with toxoplasma, and on antigens from T. cruzi grown in cell cultures.